Process and apparatus for sealing cathodic bars in a prestressed condition

ABSTRACT

The invention concerns a process and an apparatus for applying prestresses to carbonaceous cathodic blocks in the operation of sealing the cathodic bar in position with cast iron. 
     The block (1) in the course of the sealing operation is lifted on two rollers (16A, 16B) in the vicinity of its ends (E) and stresses F1 and F3 are simultaneously applied respectively to the ends (10) of the lateral edges of the limb portions (3) of the block and to the central part (C) of the block. Such stresses must be applied during the sealing operation and maintained for 15 to 30 minutes after the end of the cast iron casting operation. 
     That virtually totally suppresses the occurrence of cracks in the cathodic blocks.

TECHNICAL FIELD OF THE INVENTION

The present invention concerns a process and an apparatus for sealingcathodic bars in a prestressed condition in carbonaceous blocks whichform the cathode in electrolysis tanks for the production of aluminiumusing the Hall-Heroult process.

STATE OF THE ART

In such tanks, the cathode is formed by an assembly of parallelepipediccarbonaceous blocks which are disposed in contiguous relationship and atthe base of which has been cut one (or sometimes two) grooves in whichan iron bar (or sometimes two half bar portions) are sealed by casting aspecial cast iron. The iron bar projects at the ends of the block so asto be extended to the outside of the tank, with the ends of the iron barforming the "cathodic outputs" to which are connected the "cathodiccollectors" which carry the electrolysis current to the anodic system ofthe following tank in the series. That arrangement is virtuallyuniversal and it is described for example in French Pat. No. 1 161 632(PECHINEY) or U.S. Pat. No. 3,489,984 (BAILEY).

DISADVANTAGES OF THE PRIOR ART

The general trend is to increase the unit power and therefore thedimensions of electrolysis tanks and the intensity passing therethrough.For cathodic blocks of large dimensions, the thermal shock which isinvolved in the operation of casting the cast iron at the time ofsealing the bar in the cathodic block gives rise to mechanical stresseswhich can be sufficiently severe to cause cracking of certain regions ofthe carbonaceous block and in particular at the ends of the grooves or,in the central part, on the outside edge of the limb portions of theblock.

If a cracked block is used for making up a tank vessel, the crack tendsto be enlarged in the tank in operation of the arrangement; thatphenomenon can result in premature demise of the cathode due to theinfiltration of liquid aluminum into the crack. It is for that reasonthat any cracked block has to be rejected, which can give rise toconsiderable increases in costs.

In French Pat. No. 2 175 658 or U.S. Pat. No. 3,851,377 in the name ofSociete des Electrodes et Refractaires "SAVOIE", it is proposed that alongitudinal compression stress should be applied to the castingoperation in the parts of the block adjoining the external longitudinaledges of the limb portions, with such longitudinal compression stresssubstantially reducing the risk of sealing cracks appearing.

SUBJECT-MATTER OF THE INVENTION

The subject-matter of the invention is a process for sealing at leastone metal cathodic bar in each groove of a parallelepipedic carbonaceousblock intended to form the cathode of a tank for the production ofaluminium by electrolysis, said process comprising casting liquid castiron in the free space between each cathodic bar and each groove in thecarbonaceous block, the block during the sealing operation being sopositioned that the groove and the limb portions are directed upwardly,and put under prestress during the operation of casting the cast ironand for a period of time which is at least equal to ten minutes afterthe end of the casting operation, characterised by applying prestresseson the one hand to the two ends of the block, on the side edge of thetwo limb portions, and on the other hand, on the upper face of the limbportions of the block, at at least one point, either in the centralportion or at two intermediate points, for example at one third and attwo thirds of its length. It will be appreciated that the word "point"is not to be taken in its strict geometrical sense but denotes the zonein which the stress is applied and the extent of which corresponds tothat of the apparatus for applying said stress (head of the jack orintermediate plate of sufficient surface area to avoid punching ormarking of the cathodic block).

Further subject-matter of the same invention is an apparatus forcarrying out the sealing process, characterised in that it comprises:

a means for lifting the cathodic block by virtue of supports disposed inthe vicinity of each end of the block,

two identical means for applying lateral prestresses to the ends of thelateral edges of the two limb portions of the block, at each end of theblock, and

a means for applying vertical prestresses to the upper face of the partof the limb portions in the central part of the block.

The prestressing forces may be applied by any known means and moreparticularly by jacks (mechanical, hydraulic, pneumatic or electrical)or heavy weights.

DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 10 illustrate the mode of carrying the invention into effect.

FIGS. 1 and 2 show two types of cathodic blocks having one and two bars,

FIG. 3 indicates the regions in which the prestresses are concentratedin the sealing operation and when applying lateral prestresses,

FIGS. 4A and 4B show the effects of applying vertical prestresses to theupper face of the block, either in its central part or at two pointsdisposed at approximately one third and two thirds of its length,

FIG. 5 shows the regions for the privileged occurrence of cracks,

FIGS. 6 and 7 diagrammatically show the regions in which prestressingforces are applied, and

FIGS. 8, 9, 9A and 10 show by way of example an apparatus for generatingthe prestressing forces.

In the following specification, and including in the drawings, thecathodic blocks will be considered in the position that they occupy insealing operations, that is to say with the groove or grooves forsealing cathodic bars in position directed upwardly, being therefore inan inverted position in relation to the position that they will occupywhen mounting the cathode in each electrolysis tank where the assemblyof the cathodic bars is disposed under the carbonaceous blocks.

FIGS. 1 and 2 show two types of cathodic blocks 1 with respectively oneand two sealing grooves 2. The term "limb portion" of the cathodic blockis generally used to denote the upper part 3 which is disposed on thetwo large sides 4 of the parallelepiped which is formed by the block,above the level of the sealing groove 2, approximately above the brokenline AA'.

FIG. 1 also shows in broken lines the contour of two cathodic half barportions 5A and 5B which leave a central space of a few centimetersbetween them, as indicated at 6.

For sealing the bars (or half bar portions), closure plugs of rock woolor the like are provided at the two ends and in the space 6, and theliquid cast iron coming directly from a smelting furnace is poured,optionally after preheating of the bars and the block to a temperaturewhich may attain for example 700° C.

After cooling the blocks are turned over through 180° so that, at thetime at which they are set in position in each tank to form the cathode,the metal bars are disposed beneath the blocks.

The main cracks are formed at the end 7 of the groove, these being thecracks 8 which are referred to as "V"-shaped cracks, while on theoutside edge of the limb portions 3, generally in the central part,there are cracks which are referred to as "transverse" cracks, asindicated at 9.

To avoid the formation of cracks which appear shortly after thecommencement of cooling of the cast iron (generally in the quarter of anhour which follows the end of the casting operation), prestresses ofappropriate direction and sufficient intensity are simultaneouslyapplied to the regions in which cracks occur.

Such prestressing forces may be applied either by means of a jack (ofall types) or by means of heavy weights, or by a combination of the twomethods.

On looking at FIG. 3, it will be seen that the limb portions of theblock are subjected to antagonistic forces in the course of thatoperation. On the one hand, it is known that the stresses F_(T) due tothe thermal effects in the course of the sealing operation tend toinduce V-shaped cracks 8 by a shearing effect (traction) in the hatchedregion 8A. However the prestress F1 applied to the upper part of thelimb portion tends above a certain level to cause rupture of the limbportion in a tensile mode in the hatched area 8B.

It is therefore necessary for the prestress F1 to be very preciselyadjusted in dependence on the mechanical characteristics of the carbonforming the block, in such a way that it balances the sealing stresswhich tends to cause the cracks 8, while remaining at a lower level thanthe breaking stress of the material in tension in the zone 8B.

In practice, and with carbonaceous blocks of conventional dimensionswhich are used in 150 to 200 KA electrolysis tanks, forces F1 which mayattain 100 to 200 daN are applied in the vicinity of the lateral ends ofthe limb portions 3, at the points 10, by means of jacks. By way ofnon-limiting example, the blocks may be 500 mm in width, 450 mm inheight and 2400 mm in length, with a groove which is 160 mm in width and155 mm in height. The point 10 of application of the force is preferablydisposed in the upper third of the height of the limb portion.

The prestressing operation is effected prior to casting the cast ironand the prestressing is maintained for at least 10 minutes after the endof the casting operation and in practice for 10 to 30 minutes.

In order to prevent the formation of transverse cracks 9 on the outsideedges 11 of the limb portions 3, it is possible to apply a verticalupwardly directed force F2 under the two ends of the blocks 1simultaneously with a downwardly directed vertical thrust force F3 onthe central part of the block 1. However it is also possible and easierto place the two ends E of the blocks 1 on two rigid supports 12 whichare disposed beneath the ends thereof and to apply solely theprestressing force F3 to the upper face of the block, either at a pointlocated in the central part thereof or at a plurality of points, forexample at two points which are disposed approximately at one third andat two thirds of the length thereof. The force F2 is then formed by thereaction of the rigid supports 12.

The application of that prestressing force in opposition to transversecracking gives rise to the same problem as in respect of the V-shapedcracks. FIG. 4A shows that, if the prestressing force F3 is applied tothe two upper faces on respective sides of the central groove, it isfound that the block is subjected to a plurality of mechanical stressingforces (prestressing force F3 plus thermal stresses due to the sealingoperation) in compression (regions marked by the sign -) and in tension(regions marked by the sign +), with a maximum (indicated by the twosigns +) in the bottom part which is beneath the sealing groove. Suchstresses could also result in rupture of the block if they were toexceed the tensile breaking strength of the carbonaceous material, whichis between 2 and 5 MPa (about 20 to 50 kg/cm²).

FIG. 4B also shows the level of prestressing in the block depending onwhether F3 is applied in the central part (the solid-line curve,referred to as the "triangle" curve) or at two points F3A, F3B, atapproximately one third and two thirds of the length L (broken-linecurve, referred to as the "trapezium" curve). In the second case, theprestressing is more homogenously distributed and the risk of rupturingthe block when it is put under prestress is reduced, provided that, asin the case of the V-shaped cracks, the prestressing F3 is so calculatedas to compensate for the sealing stresses without exceeding the tensilebreaking stress of the block.

FIG. 8 is a view in central longitudinal section of the apparatus forcarrying out the invention.

The block 1 is delivered by the roller conveyor 13 to the sealingstation where it is first supported on the four rollers 14. After thecathodic bars 5A and 5B have been set in position, the block is raisedby a few millimeters by means of the pivoted assembly 15 which isprovided at its ends with two support rollers 16A and 16B.

The assembly 15 is formed by a jack 17 which is mounted on a fixed point18 and which applies on the one hand a thrust force to the arm 19 whichis pivotally mounted in its central portion at 20, and on the otherhand, a pulling force to the triangular arm 21 which is pivotallymounted on the fixed point 22. As a result of that action, the roller16A is lifted by a few millimetres and, by way of the rod 23 and thesecond triangular arm 24 which is pivotally mounted on the fixed point25, the second roller 16B is raised by a height substantially equal tothat of the first roller. The block 1 is therefore then only supportedon the two rollers 16A and 16B. The whole of the lifting mechanism isintegrated with the roller conveyor which delivers the block 1 to thesealing station.

The following forces are then applied to the block 1 (given by way ofnon-limiting example, and in the case of a block measuring 450×500×2400,as already referred to above):

4 substantially equal forces F1, applied at the ends of the limbportions 3 (see FIG. 4), at the points 10; and

2 substantially equal forces F3 which are applied to the upper centralpart (C) of each outer limb portion 3 or, in an alternative form, 4substantially equal forces F3A, F3B, at one third and two thirds of thelength of the block.

The 4 forces F1 are applied by means of the apparatus diagrammaticallyshown in FIG. 9A. A jack 26 simultaneously applies to two rigid arms 27Aand 27B which are pivotally mounted at point 28, a thrust force whichhas the effect of applying two substantially equal stresses F1symmetrically by way of the bearing members 29A and 29B.

One of those apparatuses is mounted at one end of the block 1. A secondapparatus 30A identical to 30B is positioned at the other end of theblock 1. Each of the stresses 1 is of the order of 100 to 200 daN.Having regard to the nature of the material forming the block, it willbe apparent that the forces must be applied by way of bearing plateshaving a sufficient surface area to avoid any risk of "punching" thecarbon. That comment also applies to the vertical prestressing force F3.The central part of the block 1 is subjected to the action of the system31 for producing a stressing force by means of a heavy weight.

The apparatus 31 is formed by a fixed gantry 32 on which a substantiallyhorizontal arm 34 is pivotally mounted at 33. At its end which is inopposite relationship to the horizontal-axis pivotal mounting 33, thearm 34 supports a heavy weight 35 at a distance D1 from the pivotmounting 33. On an intermediate horizontal-axis pivot mounting 36,disposed at a distance D2 (<D1), the arm also supports a stirrup 37 forapplying the vertical stress F3 in substantially symmetrical fashion tothe upper face of the two limb portions. At each of its two ends thestirrup 37 carries two bearing blocks 38A and 38B which, when the blockis lifted and the support 39 is retracted, apply the two stresses F3which are equal to the weight of the mass 35 multiplied by the ratio ofthe lever arm D2/D1. If for example the pivot mounting 36 is disposed aquarter of the way along the length of the arm from the fixed point 33,a weight 31 of 1 tonne will apply a force of 4 tonnes, that is to sayabout 4000 daN, to the limb portions. In practice that prestressingforce F3 may be between 1500 and 2500 daN (that is to say, 3000 to 5000daN in total), and preferably around 2000 daN for the type ofcarbonaceous block indicated above (measuring 500×450×2400 mm).

In an alternative form, the forces F3 may be applied by means of a jackwhich for example may be connected to a stirrup which is identical orsimilar to the stirrup 37. In addition, as indicated above, they mayalso be applied at two points, disposed at approximately one third andtwo thirds of the length of the block, by means of two identicalapparatuses 31.

After the prestressing forces F1 and F3 have been applied, it ispossible to proceed with the cast iron casting operation under the usualconditions, while maintaining the application of F1 and F3 for a periodwhich is at least equal to 15 minutes and which is preferably between 20and 30 minutes after the end of the casting operation.

If necessary, cooling of the block may be slowed down by applying panelsor blankets of heat-insulating material.

Use of the invention made it possible virtually completely to eliminatethe occurrence of the sealing cracks.

In the recent construction of a series requiring the sealing and thepositioning of more than 5000 cathodic blocks, 10 cracked blocks werefound, that is to say 0.2%, whereas previously the average rate ofcracking in the most serious cases could reach 5%.

We claim:
 1. A process for sealing at least one metal cathodic bar intoeach groove in a parallelepipedic carbonaceous block intended to formthe cathode of a tank for the production of aluminium by electrolysis,the process comprising casting liquid cast iron in the free spacebetween each cathodic bar and each groove in the carbonaceous block, theblock being positioned during the sealing operation in such a way thatthe groove and the limb portions are directed upwardly, and put underprestress during the operation of casting the cast iron and for a periodwhich is at least equal to ten minutes after the end of the castingoperation characterised by applying the prestresses on the one hand inthe vicinity of the two ends of the block, on the lateral edge of thetwo limb portions (F1), and on the other hand at at least one point onthe upper face of the limb portions of the block (F3).
 2. A sealingprocess according to claim 1 characterised in that the prestresses areapplied by a means of a jack.
 3. A sealing process according to claim 1characterised in that each prestress F1 is of a value of between 100 and200 daN.
 4. A sealing process according to claim 1 characterised in thateach prestress F3 is of a value of between 1500 and 2500 daN.
 5. Asealing process according to claim 1 characterised in that, at each endof the block, the prestresses F1 are applied by means of a single jackacting on two pivoted rods which act substantially symmetrically on thelateral edge of each limb portion.
 6. A sealing process according toclaim 1 characterised in that the prestress F3 is produced by a heavyweight disposed at the end of a lever arm and is applied substantiallysymmetrically to the upper face of the two limb portions.
 7. A sealingprocess according to claim 6 characterised in that the prestress F3 isapplied in the central part of the block.
 8. A sealing process accordingto claim 6 characterised in that the prestress F3 is appliedsubstantially at one third and two thirds of the length of the block. 9.Apparatus for carrying out the sealing process according to claim 1characterised in that it comprises:means (15) for lifting the cathodicblock including support means disposed in the vicinity of each end ofthe block, two identical means (30A, 30B) for applying lateral stresses(F1) to the ends of the lateral edges of the two limb portions (3) ofthe block, at each end of the block, and means (31) for applyingvertical stresses (F3) to the upper face of the part of the limbportions in the central part of the block.
 10. Apparatus according toclaim 9 characterised in that the means (15) for lifting the cathodicblock comprises two rollers (16A, 16B) which are spaced by a distanceslightly less than the length of the block, each roller being connectedto a triangular arm (21, 24) pivotally mounted at a fixed point (22,25), rotary movement of each arm about said fixed point being controlledby a double-acting jack (17).
 11. Apparatus according to claim 9characterised in that the means for applying the lateral stresses (F1)is formed by a jack (26) conected to one of the ends of two rigid arms(27A, 27B) pivotally mounted on a common fixed point (28), the other end(29A, 29B) thereof bearing against the outside edges of the limbportions of the block.
 12. Apparatus according to claim 9 characterisedin that the means for applying the vertical stresses (F3) to the upperface of the block comprises a weight (35) suspended at the end of asubstantially horizontal arm (34), the other end of which is pivotallymounted on a fixed point (33), and which at an intermediate point (36)supports a stirrup (37) provided in its lower part with two bearingblocks (38A,38B).
 13. A sealing process according to claim 1characterised in that the prestresses are applied by a means such as aheavy weight